In recent years the elimination of various contaminants from our environment has become important. One of the most troublesome contaminants is sulfur dioxide. Sulfur dioxide is produced by the burning of various fuels, i.e., in electric power generation, industrial, domestic and vehicular fuel use, the smelting of ores, the recovery of sulfur from a sulfur compound-bearing gas stream, oil refining, and also as an intermediate in the manufacture of sulfuric acid. Many processes have been proposed for the removal of sulfur dioxide from the off gases of these processes, particularly the off gas from sulfuric acid manufacture. Some of these processes involve the conversion of sulfur dioxide to elemental sulfur while others involve sulfur dioxide removal with a liquid sorbent, still others with a dry sorbent and finally multiple stage catalytic oxidation, or catalytic oxidation with interpass absorption. All of these prior art processes present an economic problem to the industry involved.
Assignee's copending application Ser. No. 173,247, filed Aug. 19, 1971, describes a process wherein the SO.sub.2 content of the off gases from sulfuric acid plants can be reduced to acceptable levels by incorporating a low temperature, high efficiency SO.sub.2 oxidation catalyst, e.g., supported platinum, rubidium-promoted vanadate, or a cesium promoted vanadate composition, in at least the last stage of a multi-stage catalytic converter, and subsequently contacting the off gases with an aqueous sulfuric acid solution containing H.sub.2 O.sub.2 or a peroxy acid of sulfur. Therefore a highly efficient, low temperature oxidation catalyst in the converter is important in order to reduce SO.sub.2 levels in the off gas, thus reducing peroxide or peroxy acid consumption which concomitantly permits a large reduction in size of the peroxide or peroxy acid contacting tower.
It is important to have a low temperature catalyst in at least the final stage of SO.sub.2 to SO.sub.3 conversion since equilibrium favors SO.sub.3 as the temperature is lowered. For instance, the equilibrium relationship where the inlet SO.sub.2 per cent is 10% in the range of 400.degree. to 600.degree. is pointed out below.
______________________________________ Temperature (.degree. C) % Conversion ______________________________________ 400 99.21 420 98.72 450 97.53 480 95.50 500 93.53 520 90.95 540 87.72 560 83.79 600 74.04 ______________________________________
From the above data it is thus seen that in going from a catalyst that is effective at 480.degree. C to one that is effective at 450.degree. C, the SO.sub.2 leakage in the off gases of a converter would be reduced by 45.2% (182% more SO.sub.2 leakage at 480.degree. C than at 450.degree. C). However, in going from a temperature of 480.degree. C to 400.degree. C the SO.sub.2 leakage would be reduced by 82.4% (570% more SO.sub.2 leakage at 480.degree. C than at 400.degree. C).
Whenever the H.sub.2 O.sub.2 cleanup procedure for the SO.sub.2 leakage described above is utilized, a corresponding decrease in the H.sub.2 O.sub.2 (or peroxy acid of sulfur) consumption is achieved.
U.S. Pat. No. 1,941,426 to Beardsley et al (dated Dec. 26, 1933) discloses the preparation of cesium vanadate by reacting ammonium metavanadate and cesium chloride together. The catalytic material is then placed on a carrier comprising chips of Celite. The catalytic material is stated to be useful in the method of making sulfur trioxide. However the catalyst of Beardsley et al does not have a high efficiency (see applicant's Examples 20 and 21). It is known in the art that the chloride ions poison the catalyst (see Duecker & West, The Manufacture of Sulphuric Acid, Reinhold, 1959, p 184) but most critically, applicant has found that the primary catalytic material (cesium vanadate) must be prepared in an intimate homogeneous mixture with the promoters and carrier material as opposed to impregnating the solid support with the primary catalytic material as taught by Beardsley. Applicant's procedure would seem to be in the wrong direction for the production of a low cost, low temperature, catalytic material because the expensive cesium component is intermixed with and consequently diluted by the carrier which would usurp a large fraction of the surface available for reaction. This is in contrast to the coated support of Beardsley which presents a surface entirely coated by the coating-impregnating procedural step.
Various combinations of cesium promoted vanadate catalysts in conjunction with promoters and/or activators have been disclosed in the art as being advantageous in obtaining low temperatures and greater efficiency in the conversion of SO.sub.2 to SO.sub.3 (see Duecker & West, The Manufacture of Sulphuric Acid, Reinhold, 1959; pp 171-176). However, cesium is expensive and also catalysts thus far taught in the art are inadequate in the objective at low SO.sub.2 leakage and consequently catalysts having even greater activity are desired.
The catalyst produced by the process of this invention provides such a catalyst having greater efficiency in the conversion of SO.sub.2 to SO.sub.3 and requires less expensive ingredients than previously known catalysts.